[R] expanding factor with NA

[Liaw, Andy]

Strangely (to me), just passing na.action=na.pass to model.matrix doesn't
work:

> f <- factor(rep(letters[1:3], 5))
> is.na(f[sample(15, 3)]) <- TRUE
> model.matrix(~f, data=model.frame(~f, na.action=na.pass))
   (Intercept) fb fc
1            1  0  0
2            1  1  0
3            1  0  1
4            1  0  0
5            1 NA NA
6            1  0  1
7            1  0  0
8            1 NA NA
9            1  0  1
10           1  0  0
11           1  1  0
12           1  0  1
13           1 NA NA
14           1  1  0
15           1  0  1
attr(,"assign")
[1] 0 1 1
attr(,"contrasts")
attr(,"contrasts")$f
[1] "contr.treatment"

> model.matrix(~f, na.action=na.pass)
   (Intercept) fb fc
1            1  0  0
2            1  1  0
3            1  0  1
4            1  0  0
6            1  0  1
7            1  0  0
9            1  0  1
10           1  0  0
11           1  1  0
12           1  0  1
14           1  1  0
15           1  0  1
attr(,"assign")
[1] 0 1 1
attr(,"contrasts")
attr(,"contrasts")$f
[1] "contr.treatment"

[OK, it's not so strange: na.action is not a documented argument for
model.matrix, and the call to model.frame in model.matrix.default does not
have ..., but shouldn't it?]

Andy

> -----Original Message-----
> From: Thomas W Blackwell [mailto:tblackw at umich.edu] 
> Sent: Monday, October 27, 2003 3:08 PM
> To: J.R. Lockwood
> Cc: r-help at stat.math.ethz.ch
> Subject: Re: [R] expanding factor with NA
> 
> 
> Perhaps a much simpler method (just thought of it) would be to set
> 
> options(na.action="na.pass")
> 
> before you start.  Or use  na.action=na.pass()  as an 
> argument in the call to  model.frame(), since that's where 
> the problem begins. See  help("na.omit"),  help("model.frame").
> 
> -  tom blackwell  -  u michigan medical school  -  ann arbor  -
> 
> On Mon, 27 Oct 2003, J.R. Lockwood wrote:
> 
> > I have a factor (with "n" observations and "k" levels), with only 
> > "nobs" < n of the observations not missing.  I would like 
> to produce a 
> > (n x k) model matrix with treatment contrasts for this factor, with 
> > rows of NAs placeholding the missing observations.  If I use
> > model.matrix() I get back a (nobs x k) matrix.  Is there an 
> easy way 
> > to get the (n x k) without carrying along a row ID and merging? 
> > Thanks.
> >
> > J.R. Lockwood
> > 412-683-2300 x4941
> > lockwood at rand.org 
> > http://www.rand.org/methodology/stat/members/lockwood/
> 
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